## INTRODUCTION

## METHODS

## Participants

^{2}. The development dataset included 141 subjects who participated in the “Measurement of GFR and calculation of GFR estimates for Koreans” clinical study, which was conducted to develop an ethnic coefficient for the IDMS MDRD Study equation to estimate GFR more accurately in the Korean population [15]. The validation dataset was used to validate the performance of the modified equations using Korean coefficients derived previously. A combined dataset of the validation and development data was used to derive new Korean coefficients for IDMS MDRD Study equations and to validate the performance of the modified equations with the new coefficients. The same inclusion and exclusion criteria were used for all participants in the development and validation datasets. Details of these criteria have been reported previously [15]. Participants ≥ 18 years of age who agreed with the study objectives and voluntarily provided written informed consent were included.

## Glomerular filtration rate measurement

^{2}BSA, calculated with the Dubois-Dubois formula [18].

## Serum creatinine measurement

## Ethics statement

## Statistical analyses

*p*value < 0.05 was considered significant.

## RESULTS

## Study population

^{2}in the validation and combined datasets, respectively.

## Validation of performance of the modified IDMS MDRD equations with Korean coefficients derived from a previous development study using the validation dataset

^{2}= 0.80). When eGFR was plotted against mGFR, the eGFR slopes using the modified equations (equations 1 and 2) were not significantly different from those for eGFR using the IDMS MDRD Study equations.

^{2}.

## Development of new Korean coefficients and overall performance of the combined dataset

*p*= 0.023). Analyses stratified according to sex and age showed similar results for all subgroups (data not shown).

## DISCUSSION

^{99m}Tc-DTPA) and urinary inulin clearance using the constant infusion method, respectively. Moreover, these methods are different from the renal clearance of

^{125}I-iothalamate used in the MDRD Study. Several investigators have reported that iothalamate and

^{99m}Tc-DTPA clearance overestimate GFR compared with inulin clearance [22,23]. Using the bolus injection and constant infusion methods to determine inulin clearance may lead to differences in mGFR, even though several studies showed that the difference was small and would be acceptable in clinical practice [24,25]. Therefore, discrepancies between ethnic coefficients may be due to the different GFR measurement methods used in each study. Second, strategies for recruiting, and the clinical characteristics of the participants, were slightly different in each study. Our study was an outpatient-based study, whereas the Chinese study excluded participants with muscle atrophy, and the Japanese study was primarily inpatient-based. In addition, the causes of CKD were somewhat different among the studies. More than 50% of the participants in the Japanese study were patients with glomerulonephritis, and the proportion of participants with diabetes mellitus in the Chinese study was relatively smaller than in the other studies. The Chinese and Japanese coefficients were derived from patients diagnosed with CKD, but the present study included healthy volunteers without CKD. Previous studies have reported that patients with diabetes mellitus have super-normal GFR values at an early stage [26,27]. In addition, our results and those of the other studies showed that eGFR almost always underestimates mGFR in a healthy population [26,28]. Therefore, the different characteristics of the participants in these studies may have impacted on the derivation of the ethnic coefficients. Additionally, we cannot rule out the possibility that tubular secretion of Cr may differ among ethnic groups [29].

^{125}I-iothalamate. Thus, the comparison between the original and modified equations may have been biased. Second, the size of the study population was relatively small compared with that used in previous studies of different racial and ethnic populations [5,6,11-14]. Therefore, we cannot rule out the possibility that the relatively smaller sample size may have affected the negative results (i.e., no significant difference between equations). In addition, we only included a few healthy volunteers without CKD. Considering that the difference between eGFR and mGFR was large among participants with GFR ≥ 90 mL/min/1.73 m

^{2}in this study, our findings may not be generalizable to healthy individuals with normal renal function. Additionally, our study was restricted to participants with native kidneys. Clinical presentations, such as donated or recipient kidneys, influence the performance of known GFR-estimating equations [30,31]. Therefore, findings in transplant recipients may not be the same. Finally, new coefficients from the combined dataset should be validated in larger, more diverse populations.